This study focuses on the influence of the resultant force caused by the density difference of the two phases on the droplet generation process in a step-emulsification microfluidic device. Threedimensional visualization is used to observe the characteristics of droplet generation. According to the difference in droplet generation form, it can be divided into drip generation mode and suspended generation mode. In the suspended generation mode, the droplets are more inclined to the formation process of the expansion of the ellipsoid, whose size in the horizontal direction is larger than the vertical direction; while in the drip generation mode, the shape of the droplet is opposite. The control stage of the droplet generation under various operating conditions is determined. As the viscosity of the dispersed phase increases, the control stage of the suspended generation mode is the necking stage, and the control stage of the drip generation mode changes from the first stage of three-dimensional expansion to the necking stage. The model of the droplet generation is proposed by combining mechanical analysis and experimental testing data, and the droplet size prediction formula is thereby obtained.